If made continuously, extremely long and energy inefficient curing ovens are required for a relatively slow moving product. Also, it is difficult to obtain the desired surface finish or planarity, which often requires an embossed or textured surface for cosmetic appearance only.
As a result, large plastic reinforced panels such as employed as liners for refrigerator trucks or railroad cars are usually made by a matched metal die process. When closed and filled with a reinforced plastic composite or preform, the matched die mold is subject to heat and pressure to cure the plastic. The advantages of matched die molding are principally part-to-part uniformity and the ability to produce the desired finished surfaces on one or both sides of the panel. The disadvantages are primarily cost or productivity. Moreover, matched die molding is not energy efficient.
Matched die molding for large panels requires large presses which must be opened and closed. While open, they lose heat. Moreover, such process generally requires the use of wet molding composites or preforms which must be prepared to have the proper amount of reinforcement and resin. The composite or preform must then be inserted into the open press between the dies. The press then closes and remains closed under high pressure as the panel is first formed and then cured. The energy requirements and heat losses for large size presses forming such panels are substantial. Moreover, even if such presses are equipped with substantially automatic feed and part removal devices, a matched die process is labor intensive not only in the production of the molding composites or preforms but also in the loading and unloading of the press for subsequent panel trimming and storage. Moreover, the working environment in and around the heat of the press is less than ideal.
Accordingly, a machine or process which can continuously produce large size reinforced plastic (FRP) sheet or panels with energy efficiency is desirable.
A method of manufacturing plastic panels in a continuous process is shown in U.S. Pat. No. 3,163,698. In such process the reinforcement is applied between flexible sheets which is pulled around a pressure roll and through a heater sufficient to raise the temperature of the sandwich to approximately 200.degree. C. for a period of time of about 2 minutes. Reinforced polyester sheets have also been made by direct application of chemically thickened polyester resin and chopped glass between films which pass through pressure rolls for subsequent curing as seen in U.S. Pat. No. 3,894,134.
Machines employing continuous belts which pass around a heated drum have long been employed for vulcanizing rubber and other sheet materials. Reference may be had to U.S. Pat. Nos. 3,726,627, 3,121,912, 2,434,541, 2,069,589, 2,958,096 and 1,806,811 for examples of such continuous belt vulcanizers.
Other types of machines using continuous bands or belts may be seen in Swallow U.S. Pat. No. 2,442,443 relating to the pressing of plastic sheet, Garrett U.S. Pat. No. 236,489 relating to the treating of fabrics with waterproofing materials, Kessler U.S. Pat. No. 3,241,182 relating to the production of embossed plastic mats, Einzinger U.S. Pat. No. 4,045,262 relating to the production of a laminar board with a base of bonded wood particles, and the early Weber U.S. Pat. No. 524,746 relating to the manufacture of cork board.
However, to applicant's knowledge, such machines have not been employed continuously to form reinforced plastic sheet for panels of substantial width while still obtaining dimensional accuracy, surface finish, and the proper distribution of the reinforcement and thermoplastic or thermoset materials into which the reinforcement must be evenly distributed.